Golf clubs and particularly the driver have been modified in recent years to have a so-called “spring effect.” The spring effect is such that the hitting surface (called “face”) is made to be less stiff and rigid than earlier designs. Upon head-ball impact near the face center, the face deflects within its elastic limit and it has been found that if this spring effect is optimized, the ball will travel some 5 to 15 yards farther than for previous designs. Hits that are somewhat off center do not fully realize this spring effect. As a result, in common terminology, this means the size of the “sweet spot”is undesirably small. A co-pending patent application Ser. No. 10/210,329, filed Aug. 1, 2002, shows that the rear part of the club head, called the “shell”, may be made to have much less stiffness than usual designs. The shell can thus combine with flexibility of the face so that this spring effect is also at least partly experienced by hits that are off center, meaning near the perimeter of the club face. The present invention describes an unusual mechanical design for the shell walls such that a metal shell can be made to have reduced stiffness to the desired degree. This is an important advantage because ordinary designs of metal shells having the desired stiffness would require corrugations or other features and would have more weight than can be tolerated. If made very long in the front-back dimension of the club head, a metal shell of conventional nature would be far too long for all known metals for the desired front-to-back stiffness.
The co-pending patent, Ser. No. 10/210,329, filed Aug. 1, 2002, describes a club head shell structure having acceptable weight and acceptable stiffness for hits away from the face center and particularly for hits near the perimeter of the face. One way this is achieved is by use of plastic material for the shell such as polycarbonate. It has low enough stiffness in compression for the purpose without excessive weight. It describes other ways of providing the desired stiffness, not like the present invention.
Bridge trusses and similar trusses used as floor beams are similar to the present invention in that they can store potential energy mainly in uniform compression or tension in the elements. They differ in that whereas the present invention uses a multiplicity of related structures in the direction of the applied load, trusses have no reason to have 2 or more truss structures acting on each other in the direction of the applied load. Such trusses also are more concerned with achieving high rigidity with minimal deformation under load, whereas the present invention is strongly concerned with relatively large deformation. Such trusses are not made from one piece of material, whereas this is a preferred method of construction of the present invention.
A fundamental comparison with prior art springs in general is the storage of energy per unit weight of the structure. As applied to a material having uniform stress at the elastic limit, the elastic energy stored in a cubic inch of material is sometimes called the “modulus of resilience” or as the “unit resilience”. It varies in the structure as the local stress varies and may be measured as elastic energy stored per unit weight or per unit volume. Various references such as the 8th edition of Mark's “Standard Handbook for Mechanical Engineers” show resilience for beams in bending, coil springs, and numerous other structural configurations that have non-uniform stresses. For a given value of maximum stress, they store from as little as 1/12th to ½ as much energy per unit weight as the case of uniform tension or compression stresses (stresses that do not vary over the cross section considered). The preferred form of the present invention stores nearly as much energy as for uniform tension or compression. The result is substantial weight reduction for the novel spring over prior art.